This invention concerns an apparatus for nondestructive testing of workpieces by ultrasonic energy and for measuring acoustic fields in liquid or solid materials which convert ultrasonic signals to optical signals and which latter signals are then transferred by opto-electrical devices, for instance photoelectric diodes, to electric signals.
When testing workpieces with ultrasonic pulse signals, particularly with shock wave energy, receivers are needed which operate free of resonance over a large bandwidth. It is desired that the receivers transform the ultrasonic wave amplitude, particularly the ultrasonic pressure amplitude, in a linear manner to amplitude proportional electrical signals.
For a true representation of the ultrasonic field it should be possible to achieve a useable localized resolution of the sound beam cross section.
Receiver transducers are known which operate either on the principle of a mechanical-electrical transducer (piezoelectric, magnetostrictive or similar transducer) or which utilize acoustic-optical effects.
In the case of mechanical-electrical transducers, mechanical structures are involved capable of vibrating at predetermined resonance conditions. Within the range of the resonant frequency, the pulse signal is greatly distorted and a linear transformation of very short ultrasonic pulse signals to electricl signals is not possible on account of the continuing ringing of the transducer. In order to avoid these disadvantages the transducers are severely damped by electrical or mechanical means or, alternatively, they are driven far outside their resonant freqeuncy. These measures lead to a considerable loss of sensitivity when mechanical to electrical signal transformation is utilized.
Optical methods are more advantageous inasmuch as the light beam is weightless and resonant frequencies cannot be produced when optical receiving means are used. Such methods differ as a result of the geometric configuration of the receiving mode:
1. The beam spread direction of the light and of the sound subtends a larger angle, e.g. Schlieren optics. In this case a localized resolution of the acoustic field is impossible. The acoustic field value is measured integrated over the light beam volume. PA1 2. Methods in which the beam spread direction of the sound and of the light is colinear, e.g. optical interference arrangement. In this case, the deflection of a surface resulting from ultrasonic energy is measured, see U.S. patent application Ser. No. 629,062 of W. Kaule filed Nov. 5, 1975, now U.S. Pat. No. 4,046,477 dated Sept. 6, 1977. This method makes it possible to obtain localized resolution, that is, for each point in space of the acoustic field there is a signal amplitude which is proportional to the acoustic field amplitude whereby the degree of proportionality, i.e. the linearity of transformation, is very good. The disadvantage of these methods resides in the fact that monochromatic light must be used and that the apparatus must be constructed to be insensitive to mechanical shocks and temperature changes. PA1 1. It is possible to construct a significantly smaller and more compact transducer which is no larger than the well known piezoelectric transducer test probe, yet has the advantage of exhibiting an extremely wide bandwidth since the light beam is free of inertia when compared with the heretofore known piezoelectric transducer probe. PA1 2. Ultrasonic energy receivers in which the receiving surface can be selectively predetermined as to configuration and size; for instance, by means of selectively masking the receiving surface in a predetermined pattern, a focussed receiver is obtained. This possibility enables a receiver to be adapted for different purposes in contrast with the heretofore required changing of probes. For instance, it is necessary merely to interpose different light baffles in the light path instead of changing probes. PA1 3. Receiving transducer probes which for the purposes of investigating the acoustic energy in the cross section of the acoustic beam are provided with a relatively large receiving surface adapted to be scanned in a raster by a point light source, hence obtaining excellent resolution.